Archive for August, 2011

When the hot and liquid soldering tin dissolve and penetrate onto the surface of metal, it can be defined as metal wetting. Molecule of mixture of soldering tin and copper can form a new alloy which contains part of copper and part of soldering tin. Molecule bond is the core of welding, it can decide the hardness and quality of soldering point, wetting can take place only on the copper surface which has not yet being contaminated, no oxide film due to exposure under air, at the same time, temperature should reach certain level.

2 Surface tension

Everyone knows the surface tension on the water, this tension can keep the cold water drop remain the same shape on the metal plate with grease, this is because adhesion which tend to make upper liquid expansion on the surface is smaller than its cohesion. Use warm water and cleanser can reduce the surface tension, water can soak the metal plate with grease and flow out to form a thin film, if the adhesion is bigger than cohesion will occur this situation.

Cohesion of TIN-LEAD soldering tin is even bigger than water, which can make the soldering tin as ball shape which can help to minimize the surface size, to satisfy minimum energy state requirement. Flux play the role as cleanser to metal plate with grease, additional, surface tension is highly depends on the cleaning level and temperature, only adhesion power is much greater than cohesion power, reasonable wetting can occur.

3 Alloy metal

Metallic bond among copper and tin can form grain, the shape and size of grain are determined by soldering temperature and lasting time length, hardness. Less heat during soldering can form more deliberate poikilitic texture, to form the best soldering point with hardest strength. Too much reaction time, either extremely time length of soldering or overheat or both will cause crude poikilitic texture.

In this application, all the drill hole need to be through printed circuit board, no matter if they can be used as via or component lead hole. The drawback of this technology is the through hole will take over all the precious space of each layer. Without the consideration whether that layer need electric connection.

2 Buried via:

Buried via is a kind of through hole technology use to connect the 2 or more layers of multilayer printed circuit board, buried via is among the inner layer of printed circuit board, not on the surface of printed circuit board. See below picture, there is a multilayer printed circuit board with buried via.

Compare with traditional plated through hole structure, buried via can save more space. When the signal circuit density is very high, need more holes to connect the signal layers, and need more signal circuit simultaneously, buried via technology can be applied. However, because buried via technology need more process, so higher circuit density will increase printed circuit board cost.

3 Blind via

Blind via is a kind of through hole which help to connect layer from surface to inner layer, they won’t go through the whole printed circuit board, see below picture

It is a typical blind via technology, blind via technology can be applied on both side of multilayer printed circuit board, blind via can connect both through hole and component hole which will go through printed circuit board.

Blind via can overlay each other and can make smaller which can save more space and layout more signal circuit.

Refers to SMD and connector, blind via is extremely useful because they don’t need big component hole, only need small via which can connect surface layer to inner layer. On the very density and thick multilayer printed circuit board, through surface mount technology can effectively reduce the weight and provide more space for designer.

Design performance of multilayer PCB Board is similar to most of single layer and double layer PCB Board, it can avoid too much circuit fill in the very limited space, lead to impractical tolerance, high inner layer volume, even threaten the safety of product quality. Therefore, performance specification should consider complete evaluation include thermal shock, insulation resistance, resistance welding of inner layer circuit, below content discuss the mainly aspects during the process of multilayer PCB Board design.

Dimension should be designed according to application, size of system cabine, production capability limitation of PCB Board manufacturer to choose the optimal selection. Big PCB Board has many advantages, such as less laminate, shorter circuit routes among different components, which will result in higher operation speed, moreover, each PCB Board can have more IN/OUT connection, therefore big PCB Board should be 1st choice among different kinds of application. For instance in the personal computer, most of motherboard we can see are all big PCB Board. However, design signal wire layout is not an easy task, since it needs more signal layer and inner layer connection or space, the difficulty level of heat treatment could be higher too. Designer should take various reasons into account, such as standard board dimension, dimension of manufacture facility and limitation of manufacture process. In IPC-D-322 has listed some guidance about how to choose the right dimension of Printed Circuit Board.

2> PCB Board thickness

Thickness of PCB Board is determined by different elements, such as amount of signal layer, thickness and quantity of power layer, hole quantity and diameter of through-hole as well as aspect ratio, lead length of automatic insert component and connection type. Total thickness of PCB Board is make up of conducting layer, copper layer, base laminate and pre-dip material. It is difficult to control the multilayer PCB Board, so +/-10% tolerance can be viewed as reasonable.

3> Lay-Up Structure

In order to decrease PCB Board twist into the minimum level so can have as flat as possible, the inner layer of multilayer PCB Board should be keep symmetrical. So the copper layer quantity should be even, and ensure copper thickness and copper circuit pattern symmetrical. Generally the material and structure of laminate (such as glass fiber) should parallel with the edge direction of PCB Board. Because after pressing the laminate will constrict which will cause the layout of PCB Board distorted, more liable to variability and low space stability.

However, through improve PCB design can make the bow and twist of multilayer PCB Board to minimum level. Through the dissipation of copper to ensure the structure symmetry, as well as the deep-dip material being separate evenly with reasonable thickness, through which can reach the purpose of decreasing bow and twist. PCB Board manufacture should start right from the core layer, till the outer layer of surface. The minimum thickness among 2 copper layer (dielectric layer) is 0.08mm.

According to the experience, the minimum distance between 2 copper layer, that is presoak material minimum thickness after laminate should at least 2 times of copper thickness. In other words, if the 2 adjacent copper layer thickness is 30μm, so the dielectric material thickness should be at least 2*(2*30μm)=120μm, this thickness will need to 2 piece of 1080 pre-preg material.

4> Inner layer copper

Most frequent used copper is 1oz, however, for the high intensity PCB Board, it is thickness is critical since it will has strictly impedance control. This kind of PCB Board will use 0.5oz copper, for power layer and ground layer, we should choose 2oz or even heavier copper. Etching heavier copper will decrease the controllability, it could bring difficulty to realize the tolerance pattern of circuit width and space. So it need special treatment.

5> Hole

According to the diameter of component lead or dimension of catercorner, plated through hole’s diameter normally remain among 0.028 to 0.01 inch, which can ensure the enough space and better soldering.

6> Aspect Ratio

“Aspect Ratio” is the ratio among PCB Board thickness and hole diameter. Normally speaking 3:1 can be viewed as the standard ratio, although 5:1 is also quite normal. Aspect Ratio can be determined through different elements like drilling, desmear or etch-back and plating.

2 Electric Design

Multilayer PCB Board is a high performance, high speed system. Refers to high frequency, time length of signal increase become shorter, therefore signal reflection and circuit length control become quite important. In the multilayer system, the requirement of component controllable impedance performance is very high, so electric design should follow above requirement. About elements decide the impedance value include laminate and pre-preg material dielectric, wire space of same layer, dielectric thickness and copper conductor thickness. In the high speed application, laminate sequence and signal network connecting sequence are paramount for multilayer PCB Board, dielectric parameter is critical to determine impedance, propagation delay and capacitor. The dielectric of epoxy glass laminate and prepreg can be modified through adjust content level of resin.

Epoxy resin dielectric parameter is 3.45, glass is 6.2. Through control the content level of these material, the dielectric of epoxy-glass can reach 4.2-5.3.

Relative low dielectric of prepreg material is more suitable for radio frequency and microwave circuit. In the radio frequency and microwave, lower dielectric parameter will has less signal propagation delay. In the base laminate, low dissipation factor can make the electric loss reach the minimum level.

1> First of all, take some valuable literature as reference. The most valuable literature is IPC standard or Military standard, for instance, if flexible PCB would apply on area of military/aerospace, can view IPC-6013 and IPC-2223 or MIL-P-50884.

2> Circuit parameter should be determined by package of circuit pattern, it could be helpful to draw and cut out a paper template sample of flexible circuit board. According to the flexibility and profiling template experiment can achieve the highest efficiency. Design the biggest circuit network, can make a template comprise as much circuits as possible.

3> Fix the layout drawing and circuit routes, it can decide the amount of conductor layer. Cost of circuit board increase accompany with the amount if circuit board layer. For instance, 2 double layer printed circuit board could be cheaper than 1 four layer printed circuit board.

4> Calculate circuit wire width and space according to the current load bearing capability and voltage.

5> Decide the type of base material;

6> Choose the way of how to end circuit and size of through-hole, evaluate the method of ending circuit and flexibility area, to make sure if stiffner is necessary.

7> Quit all the test method, avoid to use components out of specification to low down total cost.

Soldering is a kind of process through which the metal parts being connected by different kinds of alloy soldering tin. So the parts are capable of connected through surface molecule on the pretext of not being melted.

Soldering include SOFT and HARDEN, temperature of soft soldering below 450℃, temperature of harden soldering above 450℃. Harden Soldering usually be applied on silver, gold, steel and copper, it is soldering point is much more fastness than SOFT soldering, shearing strength is 20-30 times than SOFT soldering, these 2 heat-connecting mentioned above are all using SOLDERING, because the fundamental principle of both soldering methods are all inject the molten soldering tin into the long and narrow aperture of two clean surface of components.

Soldering ensure continuity of metal. First, if two metals connected with each other through bolt connection or physical adhesion, although it can be viewed as a compact integer, but this connection is discontinuous, sometimes if there is an oxidation-insulation film, then they aren’t even physical connection. Another drawback of mechanical connection compare with soldering is if the contact space keep on oxidation will increase the resistance value. Besides, vibration and other mechanical shock will loose the bolt. Soldering can eliminate these problems, soldering locations won’t have relative displacement, contact area won’t oxidize which can help to maintain continue conduction route. Soldering is a process during which 2 metal melted and combine with each other. When soldering tin in the state of molten, it can melt the metal where is being contacted by it, most of the time there could be a very thin oxidation film on the surface of soldering tin covered metal, flux hereby will play the role of melting this oxidation film. Soldering process normally includes:

1> Flux melt down which can help to strip off the oxidation film on the surface of metal;

2> Melt the soldering tin to make the impurity and lighter flux can float onto the surface;

3> Melt partly the metal which has connection with soldering tin;

4> Cold down and finish the fusion process.

PCB Repair need to orient root cause in the circuit board, so usually need to dissemble components from circuit board and do the necessary test and measurement, this process will include:

1> Dissemble special components;

2> Component test;

3> Replace those defected components;

4> Inspect circuit pattern functions;

Dissemble and replace other components will need to instruct soldering procedure.

PCB (Printed Circuit Board): It appears in almost every electronic device. If there exist electronic components a certain kind of equipment, which are set in various sizes of PCB. In addition to fix a variety of small parts, the main function of PCB is to provide electrical connections among the parts. As electronic devices become more complexity which requiring more and more electronic components, the intensity of circuit pattern and electronic parts on the PCB are also increasing simultaneously. Bare board (without any electronic components on it) is also often referred to as “Printed Wiring Board (PWB) “. The substrate of PCB is composed of insulated as well as non-pliable material. Small circuit wire as you can on the surface of laminate is copper foil, copper foil is covered on the surface of raw laminate, and being etched away in the manufacturing process, those part being left behind as a network of fine lines form the circuit pattern. These lines are called conductors (conductor pattern) or said wiring, which are used to provide electrical connections on the PCB for those components.

1> IPC-ESD-2020: United Standard, developed by electrostatic discharge control programme, includes all the necessary design, establishment, installation and maintenance of electrostatic discharge programme. According to the history experience of some military and commercial organization, provide guidance of solution and protection during sensitive period of electrostatic discharge.

2> IPC-AC-62A: Handbook of water clearing after assembly, describe composition of residue and remains, type and feature of cleanser, process of clearing, related equipment and technology, quality control, environment control and employee safety, method of verify the level of clearness and charge of clearing.

5> IPC-7525: Template Design Directory. Provide guidelines for design and manufacture of stencil used as template of solder and surface mount bonder, also discuss template design for the application of surface mount technology, introduce hybrid-technology of thru-hole and upside-down chip mounting, include overprint, mackle and stagewise template design.

6> IPC/EIA J-STD-004: Requirement Specification of Flux, include the qualification and category of colophony, resin, Classify organic and Inorganic flux according to the amount of halide and activation level; also include the usage of flux, material which contain flux and no-residue flux when apply in the no-clean process.

A wide variety of inspection equipment has been invented in order to fulfill the task of Printed Circuit Board Inspection. Automatic Optical Inspection (AOI) is mainly applied on the inner layer inspection before outer layer Printed Circuit Board being laminated onto it. After lamination, X-Ray monitoring system being used to inspect the preciseness of layer-to-layer alignment and minor defect; Laser scanning system can help to inspect PAD before reflow soldering. All the methods mention above plus visual inspection on-site as well as automatic pick & place machine can effectively ensure the stability of final assembly and PCBA.

However, final inspection upon Printed Circuit Board Assembly is indispensable even if all the efforts be made; maybe it is the most critical step because it is the final unit for product and process evaluation.

Final inspection of Printed Circuit Board Assembly can be executed through manual method or automatic system, and most of time these 2 methods should be instructed simultaneously, MANUALLY means one operator inspect Printed Circuit Board Assembly visually through visual tool in hand and make the right judgment about the defect on the Printed Circuit Board Assembly. Automatic system use computer assistant drawing analyzing method to spot the defect; people also thinks that automatic system include all the inspection methods except manually visual inspection.

X-Ray technology provides a method which can help to assess the thickness, dissipation, cavity, crack, loose weld. Ultrasonic can inspect cavity, crack and break lead. AOI can assess cosmetic features, such as bridge-connection, tin volume and shape. Laser inspection can provide 3D pictures for cosmetic features. Infrared inspection can compare an existing good quality soldering point with the inspected location, through the thermal signal difference can find the inner defect of soldering points.

There is another important point, we have already known that all the automatic inspection technology can’t spot all the defects on the Printed Circuit Board Assembly. So manual visual inspection combine with automatic inspection could effectively solve this problem especially for those small quantity application. X-Ray inspection plus manual visual inspection could be the best combination of Printed Circuit Board Assembly inspection method.

Below are the main defects which has shown the highest frequency of existence on the Printed Circuit Board Assembly:
1) Component Missing;